1. Field of the Invention
This invention relates to an ink-jet recording apparatus and recording method which can be applied to a recording apparatus, such as a printer, a facsimile apparatus, a word processor, a copying apparatus or the like. More particularly, the invention relates to an ink-jet recording apparatus and recording method for controlling the temperature and the amount of ink discharge of a recording head.
2. Description of the Related Art
Recording apparatuses in which recording is performed on a recording medium, such as paper, an OHP (overhead projector) sheet or the like, have been practically used by mounting recording heads of various kinds of recording methods. The recording methods comprise, for example, a wire dot method, a thermal printing method, a thermal transfer method, an ink-jet method, and the like. In the ink-jet method, recording is performed by discharging ink from a recording head onto a recording medium. This method has attracted notice as a quiet recording method with a low running cost.
However, the amount of ink discharge in the ink-jet method generally has temperature dependency, so that the recording density changes depending on the ambient temperature and the like. This is because the physical properties, particularly the viscosity, of ink change depending on the ambient temperature. At low temperatures, it is difficult to discharge ink because the viscosity of the ink increases. On the other hand, at high temperatures, ink can be easily discharged because the viscosity of the ink decreases. Accordingly, in order to provide an appropriate amount of ink discharge, various methods have been adopted for controlling the temperature of the recording head.
As disclosed in U.S. Pat. No. 5,339,098, two principal control methods are considered for controlling the temperature of the recording head. In one of the methods, a heater provided at the recording head is used as control means such that the recording head is heated to an appropriate temperature at a low temperature. In another method, an ink discharging heater is used as control means such that ink is preliminarily heated at a low temperature, or the heat when the ink is discharged is controlled.
In a control method of these control means, it is theoretically desirable to detect the temperature of the recording head to be controlled, and to perform feedback control in a closed loop. In this case, however, problems often arise in the detection of the temperature of the recording head. In general, the temperature of the recording head is detected using a thermistor, a thermocouple or the like as a temperature sensor in which the value of electrical resistance or an electromotive force changes depending on the temperature. At that time, the relationship between a reference temperature and the output value of the temperature sensor differs for each temperature sensor due to variations in produced temperature sensors. Therefore, some kinds of countermeasures must be taken in order to detect an actual temperature.
In a simplest approach, only temperature sensors whose outputs are within a predetermined range with respect to a reference temperature are selected and are used without performing adjustment. This approach, however, has a great disadvantage in that the cost of the temperature sensor very much increases. In another approach, temperature sensors are ranked according to output values with respect to a reference temperature, and ranked temperature sensors are used by changing a conversion reference in accordance with a rank. Such an approach, however, also requires a considerable number of operations, such as measurement of ranks in the production process of recording heads, display of ranks, manual adjustment in an assembling process of apparatuses, mounting of means for automatically identifying ranks in a recording apparatus, and the like.
Accordingly, in general, the temperature of a recording head is controlled by performing open-loop sequence control without detecting the temperature of the recording head.
In the open-loop sequence control, it is desirable to perform thermal analysis of the recording head to be controlled, and to perform control based on the result of the analysis. Actually, however, it is rather difficult to realize such an approach. Practically, data relating to the temperature and the amount of ink discharge of the recording head to be controlled when various kinds of control inputs are provided to control means of the control system is obtained, and control is performed based on the obtained data. The obtained data have different values depending on temperature characteristics of the recording head. Accordingly, for example, control inputs are set to average values of respective recording heads by statistically processing the data.
However, the above-described conventional approach has the following problems.
When setting the control inputs to average values of respective recording heads, although no problem arises when only small variations are present in temperature characteristics of produced recording heads, the objects of control will not be achieved and recording heads will, in some cases, be operated under worse conditions when large variations are present in temperature characteristics of recording heads. In such a case, it is necessary to control temperature characteristics of recording heads by an approriate method.
In another approach, taking into consideration variations in temperature characteristics of produced recording heads, control inputs are set to small values so that the recording heads are not operated under worse conditions irrespective of temperature characteristics. In such a case, however, little control effects will, in some cases, be obtained for certain recording heads because the control inputs are set to small values.